The present invention relates to an electromagnetic wave navigation system and more particularly to a Loran-C signal receiving apparatus used in a Loran-C navigation system, which employs hyperbolic electromagnetic navigation techniques.
A Loran (Long range navigation)-C system employs a chain comprising one master transmission station and two or more subsidiary transmission stations. The master transmission station transmits a group of nine Loran pulses. The other subsidiary transmission stations transmit groups of eight Loran pulses. Each transmission station generates the above-described Loran pulses at a pulse group repetition rate prescribed for each chain. In addition, each subsidiary transmission station generates its station Loran pulses with a coding delay with respect to the transmission pulse of the master transmission station distinct from that of other subsidiary transmission stations. Hence, in the Loran-C signal receiving apparatus, the difference in distance to each of the two fixed points represented by the master and each subsidiary transmission station can be derived from the delay in time between receipt of the secondary station pulses and of master station pulses so that the location of the Loran-C signal receiving apparatus can be identified by the intersection of the two Loran hyperbolics drawn for the two sets of two fixed points.
In the Loran-C receiving apparatus, the phase of a specified cycle of the carrier wave in each received pulse (generally, the third cycle of the carrier wave) is tracked by reference to a pulse generated within the receiving apparatus and synchronized with each received pulse in order to measure the reception delay time of the pulses from the secondary transmitting stations with respect to those from the master transmission station on the basis of the phase tracking point (i.e., third cycle of the carrier wave of each Loran pulse).
Conventionally, the above-described phase tracking has been carried out by the use of PLL (Phase-Locked Loop) circuitry, e.g., shown in FIG. 1.
The PLL circuitry shown in FIG. 1 (used for the phase tracking) is disclosed in a Japanese Utility Model Registration Unexamined Open No. Sho. 58-182167 filed on May 31, 1982.
In FIG. 1, a tracking pulse 100 derived from a frequency divider 10 is sent to a phase comparator 12 (comprising an Exclusive-OR gate) in which the phases of the tracking pulse 100 and the carrier wave of the Loran-C pulse are compared. An output signal indicative of the results of this comparison (comparison signal) is sent to a VCO 16 (Voltage Controlled Oscillator) 16 via a LPF 14 (Loop Filter). The oscillating output of the VCO 16 having a frequency adjusted according to the comparison signal is sent to the frequency divider 10 to close a loop.
If the amplitude of the received electrical signal is too low, phase comparison by the phase comparator 12 is impossible. As a result, the PLL circuitry shown in FIG. 1 works in a so-called free run state, the VCO 16 accumulates frequency errors and the comparison signal (i.e., tracking pulse 100) of the phase comparator 12 gradually deviates from the tracked carrier wave of the Loran-C pulse. Consequently, once the comparison signal inputted to the phase comparator 12 deviates at least one-half cycle from the tracked carrier wave of the Loran-C pulse, a so-called cycle slip phenomenon (deviation of the tracking point by one or more wavelengths of the carrier wave) occurs, which causes a large measurement error in the location.
To cope with this problem, a temperature-compensated crystal oscillator is used in the VCO 16.
However, since the frequency error of the above-described crystal oscillator is generally one ppm (parts per million) at the maximum and the carrier wave frequency of the Loran-C pulse has a frequency of 100 kilohertz (kHz), the cycle slip still occurs if the PLL circuitry runs free for five seconds or more.
Hence, if the received electrical signal should weaken, as sometimes occurs when the Loran-C signal receiving apparatus is used for measurement of the location of a moving object such as a vehicle, the cycle slip will inevitably occur and consequently large measurement errors in the location often result.
As described above, there is a problem in the conventional Loran-C signal receiving apparatus in that when the amplitude of the received (Loran-C) signal is too low, the PLL circuitry runs free, the frequency errors in the pulse tracking the received signal accumulate, and the cycle slip phenomenon consequently occurs so that accurate measurement of location cannot be achieved.